Growing a crystal of a group III-V compound, such as GaN crystal, on a substrate of a different material from the crystal material, such as a sapphire substrate or a silicon (Si) substrate, causes stress between the crystal and the substrate due to differences in properties such as their crystal lattice constants and thermal expansion coefficients, leading to warps and cracks; thus, the process does not yield group III-V crystals of good quality.
In view of this problem, a method has been carried out for alleviating the stress between the crystals and the substrate by depositing a film of a silicon oxide (such as SiO2) on a sapphire substrate; patterning the silicon oxide film is by a technique such as photolithography, and thereafter growing a group III-V crystal onto the patterned substrate. Such a method, however, is problematic in that it requires the patterning of the silicon oxide film, which means the manufacturing cost is high.
Another technique that has been proposed is one in which a GaN layer is grown on a substrate such as sapphire by a metal organic chemical vapor deposition (MOCVD) technique, followed by the depositing of a metal film thereon and performance of a heat treatment to form voids in the GaN layer; thereafter, a GaN crystal is grown. (See Japanese Unexamined Pat. App. Pub. No. 2002-343728, for example.) Nevertheless, a problem arises with such a method because growing a GaN layer by MOCVD leads to extremely high manufacturing costs.
Still another technique that has been proposed is one in which a metal film is deposited on a sapphire or like substrate and thereafter a GaN crystal is grown. (See Japanese Unexamined Pat. App. Pub. No. 2002-284600, for example.) Such a method, however, is problematic in that the qualities of the resulting GaN crystal are compromised because the GaN crystal is grown on a metal film that has a different lattice constant from that of the GaN crystal.